There is a great desire to incorporate capacitive touch sensors with multi touch capability into hand held devices such as mobile smart phones, MP3 players, PDAs, tablet PCs, etc. Such devices often have a transparent front cover sheet that is made of glass or plastic onto the rear of which a two-layer transparent capacitive sensor is bonded. Such an arrangement can lead to a cover/sensor module that is undesirably thick and heavy and hence there is to a requirement to reduce the thickness of the 2-layer sensor as far as possible while still retaining good projected touch performance. Sensor thicknesses of 0.2 mm or less are desirable.
Prior art in the 2-layer sensor area generally involves making the transmit (Tx) and receive (Rx) electrode layers on separate substrates and then laminating them together. The two substrates for the sensor are generally made of plastic. In this case, to make a laminated 2-layer sensor with overall thickness or 0.2 mm or less would require use of polymer substrates of around 0.075 mm or 0.05 mm thickness which are difficult to handle.
Other prior art in the 2-layer sensor area involves depositing two similar transparent electrically conducting (TC) layers on the opposite faces of a single transparent glass substrate and then creating the Tx and Rx electrodes in the layers using direct write laser patterning. However, this process is limited in minimum substrate thickness as problems with the laser beam patterning the TC on one side damaging the TC on the other become very severe with glass substrate thicknesses less than about 0.4 mm.
Patterning of the TC layers on 2-layer touch sensors is generally carried out using lithography processes involving application of resist, exposure through a mask, resist development, chemical etching of the TC layer and finally resist stripping. Such multi-step processes which have to be repeated for every material layer requiring patterning have a high cost associated with them as large quantities of capital equipment are needed and large amounts of chemicals are required. A major factor contributing to the high cost of ownership is that for each sensor design special costly masks are required for every layer to be patterned. Laser direct write patterning overcomes all these drawbacks and hence there is a strong desire to use laser patterning processes but at the same time be able to make a 2-layer sensor on a single thin substrate. The two layers are provided on opposite sides of the substrate and are thus both exposed (rather than be covered by a further layer and thus sandwiched between two layers).
The present invention aims to address the above problems by providing a method and apparatus for direct write laser patterning coatings on opposite sides of a substrate without appreciable damage to the first coating when the pattern in the second coating is being formed.